This invention relates to digital telephony and more specifically relates to coordinating the transmission of data in digital telephony.
Digital telephony, the transmission of voice over digital communications systems, requires coordination of a head end unit to a remote unit. However, the reference clocks at the head end unit and remote unit are, in general, produced from asynchronous sources. For example, the reference clocks at both the head end unit and remote unit may be generated by local crystal oscillators, each of which is free running. Due to manufacturing uncertainties, the frequencies of these two oscillators (and correspondingly the head end unit and remote unit reference clocks) will not match.
There is typically a frequency offset of several hundred parts per million (ppm) between the head end clock and remote clock. In addition to a constant offset, there is the possibility of jitter on either of the generated clocks. Hence, the head end unit (also called the central office, or CO) operates at a different frequency than the remote unit (sometimes referred to as the customer premises equipment, or CPE). The frequency offset and jitter interfere with proper communication of data.
In the past, the frequency offset between the head end and remote clocks has been handled by sending both voice and non-voice data in flagged packets. Many of the packets include no voice data. The packets with both voice and non-voice data are sent with a prefix or flag that indicates the presence of voice data. In response to the flag, the receiving unit switches the voice data to a voice data buffer memory. After some voice data has been stored in the buffer memory, it is processed by voice processing circuitry. The use of a flag to indicate the presence of voice data reduces the bandwidth available for data transmission. The processing required to handle the voice data in response to the flags requires delays and expensive processing equipment. This invention addresses these problems and provides a solution.
U.S. Pat. No. 5,479,457 (Dec. 26, 1995) describes clock smoothing. However, it does not teach any clock relationships between a central office and customer premises equipment.
Further limitations and disadvantages of conventional and traditional approaches will become apparent to one of skill in the art, through comparison of such systems with the present invention as set forth in the remainder of the present application with reference to the drawings.